It seems that most of us have gotten so used to the world of the internet that we even send out our party invitations this way. Which is great for close friends and family invites, but I still like the old fashioned way better. I love seeing what kind of unique invitation the person has used, and it gives me an idea to the theme of the event as well,(which can usually help you pick out a gift).
But if you must send out online inites i suggest you use E-vites, they offer a lot of different designs and you can have everyone rsvp with their count of how many are coming, you can even set it up for people to pick what item they want to bring with them if you are doing a potluck.